This invention relates to a method for inspecting an optical fiber to detect defects of the optical fiber at the time of fabrication.
Conventional methods for inspecting optical fibers--glassy optical fibers or crystalline optical fibers--can only be applied to separated optical fibers with both ends free. An inspecting light enters one free end and exits from the other free end of the optical fiber. When a light is introduced into one end of an optical fiber, the light propagates through the optical fiber to the other end. However if there are some defects in the optical fiber, some portions of the light scattered by the defects return to the incident end. The distance between the incident end and the defect spot can be calculated by the delay time of the back-scattered light.
Another conventional method of inspection is to introduce the inspecting lights from the side surfaces of optical fibers and to analyze the pattern of the forward-scattered light or the back-scattered light. The pattern of scattered light, which is defined as the power distribution of light along a diameter of an end surface of an optical fiber, represents the fluctuation of the refractive index of the optical fiber. The distribution of defects is known by the fluctuation of the refractive index.
The first conventional method is a suitable method to inspect the defects in optical fibers longer than tens of kilometers, and the second conventional method is an advantageous method to inspect a distribution of the refractive index of an optical fiber in detail. However, there has been no suitable method for inspecting defects of the optical fibers which have just been fabricated. The conventional methods require a detector and a light source to be placed at both free ends of the optical fiber, but the optical fiber which is being fabricated has only one end. The conventional method cannot be applied on the fiber without both free ends.
Glassy fibers now form a main current of optical fibers and have been greatly improved. However, crystalline fibers are being developed now. Crystalline fibers are subjected to occurrences of local defects on fabrication accidentally or inevitably.
Thus, an easy and reliable method for discovering local defects of optical fibers on fabrication has been desired earnestly.